The invention relates to a piezoelectric drive, in particular for the generation of rotational and translational movements which can be carried out continuously or stepwise.
The inventive motor can be employed in automation systems, in robot technology, as a drive for microscope tables, for fine-positioning of various types of coordinate tables, in optical and laser systems, as well as in numerous other devices in which translational movements with high precision accuracy are required.
Piezoelectric motors or drives which are based on the utilisation of acoustic transducer travelling waves have been known for a longer period, with reference being made here for example to EP 0 475 752 and U.S. Pat. No. 5,596,241. Such motors, however, have the drawback that it is not possible to manufacture them as miniature drives, because the minimum length of the waveguide of these motors must be a multiple of 6xcex to 10xcex. In addition, the manufacture is complicated and expensive.
Linear piezoelectric motors which utilise standing acoustic waves are also known, e.g. from U.S. Pat. No. 5,453,653.
Such motors are relatively small and their manufacture is simple. A monolithic plate-shaped piezoelectric oscillator with a long and a short side and with a friction element which is arranged on one of its small surfaces is used as the drive element in such motors.
One of the large surfaces of the piezoelectric oscillator carries a first and a second electrode group. On the second one of the oscillator surfaces a continuous electrode is arranged. Each of the first and the second electrode group represents two equally sized diagonally arranged rectangular areas of the metallised piezoelectric ceramic surface. The source of the electric excitation of acoustic oscillations directs the voltage to the continuous electrode and to the first or second electrode group.
Due to the asymmetric configuration of each of the electrode groups with respect to the longitudinal axis of the oscillator the electric source voltage generates an asymmetric deformation in the oscillator plate. This results in the friction element performing a movement on a closed path. Depending on which electrode group the electric voltage is applied, the friction element moves in a forward direction or in the opposite direction. The moving friction element causes a movement of the pressed-on element. The operating frequency of the motor is in the vicinity of the resonance frequency of the second oscillation mode of the flexural oscillations of the oscillator along the oscillator length.
It is disadvantageous with such motors that an asymmetric deformation of the oscillator plate is required for the generation of acoustic oscillations. Such a motor has trajectories which differ substantially from points on the function face of the oscillator. This leads to a substantial difference in the tangential components of the oscillation velocities of these points. The latter causes instability of the movement velocity of the driven element, which is highly dependent on the real contact site of the surface of the driven element with the function face of the driven element.
Moreover, a great difference in the tangential components of the oscillation velocities causes different degrees of wear of the function face of the friction element. This renders the motor operation instable over a longer operating period.
With velocities above 0.1 m/s the non-uniformity of the movement velocity of the driven element of known motors reaches approx. 50%. With lower movement velocities, i.e. below 0.01 m/s, the inaccuracy amounts to 80% and more. Such a non-uniformity limits the application range of the motors and complicates the construction of electronic velocity stabilisers, in particular for the range of very small velocities.
Moreover, high exciter voltages are required for such motors.
The construction of a motor according to U.S. Pat. No. 5,453,653 comprises only one friction element on the surface of the piezoelectric oscillator. This renders the oscillator mechanically instable which, with high movement velocities of the driven element, reduces the positioning precision and leads to complicated constructions.
Moreover, the use of only one friction element limits the maximum possible force developed by the motor with only one piezoelectric oscillator. In known motors, this force amounts to approx. 10 N which is insufficient for many applications. The use of several oscillators combined to one packet in turn limits the positioning precision of the driven element.
It is therefore the object of the invention to specify a piezoelectric drive or motor, respectively, which comprises a uniform movement velocity of the driven element at high and low velocities, which has a higher operating stability over a longer operating period of the motor, requires a low excitation voltage, develops a high force, has a stable oscillator construction, and comprises a means or device for tracking the oscillator resonance frequency.
The solution of the object is achieved with a subject as is described in the valid main claim, with the dependent claims comprising at least suitable embodiments and developments.
According to the invention the first and the second electrode group represent two areas of identical configuration which are located on the two large opposite metallised surfaces of the plate-shaped piezoelectric transducer or oscillator (of the piezoelectric plate). Each of the two electrode groups forms at least one independent generator of non-connected acoustic standing waves which propagate along the long side of the piezoelectric oscillator or of the plate, respectively. The first electrode group forms a longitudinal wave generator and the second one a flexural wave generator of acoustic waves. The source of the electric excitation of acoustic oscillations comes from a basic generator which is electrically connected with signal inputs of a two-channel power amplifier. Each output of the two-channel power amplifier is electrically connected with the corresponding electrode groups.
Due to the fact that with the proposed motor the first electrode group forms at least one independent generator of standing acoustic longitudinal waves and the second one forms at least one independent generator of standing acoustic flexural waves and that these are designed in such a manner that no connection exists between them, i.e. that the waves produced by the generators do not influence each other, the waves propagating in the piezoelectric oscillator are pure longitudinal and pure flexural waves.
Such waves lead to pure elliptic trajectories of the oscillator body and have virtually an identical shape with hardly differing amplitudes in the areas of the defined maxima of standing flexural waves.
This enables a movement of the points in these areas on the function faces of the friction elements with virtually the same velocity. All of these elements together therefore enable a considerable increase in stability both with low and high movement velocities of the driven element.
The configuration of the electrode groups is realised in such a manner that the generators of acoustic longitudinal and flexural waves fill the entire volume of the piezoelectric plate. This reduces the exciter voltage in an advantageous manner.
With the proposed motor the first electrode group represents rectangular areas of the metallised surface of the plate-shaped oscillator. Here, the height is equal to the width. In between a unidirectionally polarised piezoelectric ceramic is disposed in a normal (vertical) direction relative to the electrodes. The electrodes are located at the sites of oscillation velocity nodes of the standing acoustic longitudinal wave which propagates in the oscillator. Such an electrode construction which forms a generator of acoustic longitudinal waves enables the generation of pure longitudinal waves in the oscillator plate.
The second electrode group also represents a rectangular area of the metallised surface of the plate-shaped piezoelectric oscillator. The height is again equal to the width. In between a unidirectionally polarised piezoelectric ceramic is disposed in the normal direction (i.e. vertically to the electrodes). The electrodes are again located at the sites of oscillation velocity maxima of the standing acoustic flexural wave which propagates in the oscillator and have insulating areas or spacings along their longitudinal axes.
Such an electrode construction allows the generation of pure flexural waves.
In a further embodiment of the proposed drive t he second electrode group represents rectangular areas of the metallised surface of the plate-shaped piezoelectric oscillator, with the height being equal to the width and with a heterodirectionally and with respect to the longitudinal axis of the oscillator symmetrically polarised piezoelectric ceramic being arranged in a normal direction between these areas or at the sites, respectively, of oscillation velocity maxima of the standing acoustic flexural waves which propagate in the oscillator.
Such an electrode construction enables the design of the generator electrodes of the acoustic flexural wave without insulating spacings in between, which increases their efficiency.
With the inventive solution of the drive the friction elements are formed as thin strips of a hard abrasion-resistant substance, e.g. oxide ceramic, metal ceramic, or of a combination with other materials. The friction elements are located in areas of oscillation velocity maxima of the acoustic flexural wave which propagates in the plate-shaped piezoelectric oscillator or in the oscillator plate, respectively.
Such an embodiment leads to a homogeneity of the tangential component of the oscillation velocity on the function face.
In a further version of the motor the friction elements are arranged on a smaller lower surface of the plate-shaped piezoelectric oscillator. This design version of the motors permits the application of several friction elements whereby the force generated by the motor or the force transmission, respectively, can be improved. In a supplementary construction version of the motor a friction element is arranged on the small lateral surface of the plate-shaped piezoelectric oscillator.
With this, a maximum possible velocity of the driven element is achieved.
The friction elements of the motor may have a two-layer structure.
The first layer of this structure is made from a hard abrasion-resistant material with a high friction factor compared to the friction layer of the driven element. The second layer consists of a hard porous material.
Both layers are joined by bonding in the sintering process. In the interface area of the first and the second layer a so-called transition layer may be formed.
With this design version of the motor a combination of the temperature co efficient differences of the piezoelectric ceramic and the material of the friction element is possible, and an increase in strength of the bonded joint of the friction element with the piezoelectric ceramic surface can be achieved.
The friction elements may be joined by means of a special bonding agent with the surface of the piezoelectric oscillator with the bonding agent chemically reacting both with the piezoelectric ceramic and with the material of the friction element by means of a low-melting lead-containing glass.
With the proposed drive the friction elements can also be formed by glass strips which are fused onto the surface of the plate-shaped piezoelectric oscillator. The glass is blended with a powder of a hard, wear-resistant material, e.g. aluminium oxide, zirconium oxide, silicon carbide, titanium carbide, or similar materials or their mixtures. The strips are located at the sites of defined maxima of the oscillation velocities of the acoustic flexural wave. With this design version a maximum possible friction factor in the friction element and the friction layer of the driven element is achieved.
In the described drive the monolithic plate is equipped with at least one fixing element in order to mechanically fasten the plate.
In the various design versions these fixing elements may be designed as rectangular, triangular, semi-cylindrical prisms, as conical, pyramid-shaped, semi-spherical elements, or in the form of rectangular elements with profile grooves or as round elements, respectively, with profile holes. The elements are located at the sites of oscillation nodes of the standing acoustic longitudinal wave which propagates in the oscillator and are rigidly connected with its surface. Thereby, a high positioning precision is achieved. The fixing elements can be made from such materials whose modulus of elasticity is approximately identical with or slightly higher than the modulus of elasticity of the piezoelectric ceramic of the plate-shaped piezoelectric oscillator. Thereby a high overall strength is achieved.
The fixing elements can also be made from materials whose modulus of elasticity is much smaller than the modulus of elasticity of the piezoelectric ceramic. This reduces the force resulting from the acoustic oscillator oscillations between the fixing elements and the surface of the piezoelectric oscillator.
In a version of the motor the fixing elements and the plate-shaped piezoelectric transducer can be manufactured from the same type of piezoelectric ceramic.
There is also the possibility to manufacture the fixing elements or parts of same from porous oxide ceramic or another porous material, which enables a good bonded joint of the fixing element with the surface of the piezoelectric plate.
With the claimed drive the fixing elements can also be designed as bending resonance plates. With this design the damping introduced by the fixing elements into the resonance system of the piezoelectric plate becomes minimal so that the energy losses in the piezoelectric oscillator can be reduced.
The fixing elements can be joined with the piezoelectric ceramic surface by means of an organic adhesive. This allows the use of materials with different temperature coefficients.
In a further embodiment of the drive the friction layer of the driven element is made in the form of a ceramic oxide layer with a thickness which is at least five times smaller than half of the wave length of the acoustic standing longitudinal wave which propagates in the oscillator. This can suppress undesired oscillations.
In a further version the body thickness of the driven element located below the friction layer is greater than half of the wave length of the standing acoustic flexural wave which propagates in the oscillator. In this case the generation of an acoustic flexural wave in the body of the driven element is nearly completely precluded.
Likewise, it is possible to arrange a damping layer between the body of the driven element and its friction layer, which is manufactured from organic materials or hard porous materials or combinations of such substances, respectively. This reduces the acoustic coupling and eliminates a thermal incompatibility due to different expansion coefficients of the friction layer and the driven element.
According to the invention the driven element may be designed in the form of a bar with a rectangular, polygonal, or round cross-section. It can, however, also be formed as a tube.
The driven element can also be formed from a hard material in the shape of a bar with a rectangular cross-section, in the body of which damping grooves are formed at periodic spacings.
If the driven element is manufactured from a hard porous material the longitudinal waves developing in the body of this element are dampened in an advantageous manner. Likewise, the pores of the driven element can be filled with a sound-absorbing filler material so that damping of both the longitudinal and the flexural waves is effected.
A further constructive version of the motor includes at least two plate-shaped piezoelectric oscillators or transducers which are arranged opposite each other and lie in one plane, with these having at least two friction layers which are parallel to each other and which are located on the two opposite sides. Here, a support bearing can be omitted.
The use of at least three plate-shaped transducers or at least three friction layers which are parallel to each other and lie in at least three planes is also conceivable. Here, the driven element is very stable in the plane perpendicular to its longitudinal axis.
In an embodiment of the motor the driven element is designed as a rotating body. It is also conceivable to arrange the fixing elements of the piezoelectric transducers in a stationary girder.
Another version with respect to construction is to arrange or place the fixing elements of the plate-shaped piezoelectric oscillator or transducer in a flat spring-type girder whereby transverse displacements of the driven element can be compensated and thus an increase in the uniformity of the movement be effected.
With respect to the control side the output of a basic generator is connected with one of the signal inputs of a two-channel power amplifier via a phase shifter. Thereby, the shape of the trajectories of points of the working surface of the friction elements can be optimally adjusted.
In a further circuit engineering version each channel of the two-channel power amplifier is formed as a bridge power amplifier with two half-bridge amplifiers and two exciter channels including phase controllers. Thereby, an electronic control of the movement velocity of the driven element can be effected.
In addition, it is possible to provide a signal level transducer and a zero signal detector with respect to the control signal in order to drive the motor with a unipolar voltage.
The piezoelectric transducer, i.e. the plate-shaped oscillator can be provided with a sensor for the detection of the longitudinal component of mechanical stresses. This sensor is disposed in one of the velocity nodes of the longitudinal wave which propagates in the plate-shaped piezoelectric oscillator. Thereby, longitudinal stresses can be determined and thus the oscillation velocity of the longitudinal wave which propagates in the oscillator be determined.
In addition, the plate-shaped piezoelectric transducer can be provided with a sensor for determining the bending component of the mechanical stresses. This sensor is located in one of the oscillation velocity maxima of the flexural wave which propagates in the transducer.
The generator, i.e. the source of the electric excitation of acoustic oscillations can be equipped with a phase detector for determining the signal originating from the sensor of components of mechanical stresses. The phase detector includes support and phase measuring inputs and an output, with the basic generator having an input for the electric frequency control. The support input of the phase detector is electrically connected with one of the electrode groups. The measuring input of the phase detector is electrically connected with the sensor for determining the mechanical stresses. The output of the phase detector is electrically connected with the input for the electrical control of the excitation frequency of the basic generator. With such a circuit arrangement an exact tracking of the mechanical resonance frequency of one of the oscillation types of the piezoelectric transducer can be effected in order to further""stabilise the motor operation.